sqlState normalized (error object) && minor version bump && Changelog update

[williamdes]

Changed sqlState of an error to a string of 5 chars as mysql specifies it [here]
(https://dev.mysql.com/doc/refman/5.5/en/mysql-sqlstate.html) and to align with mysqljs/mysql.

I updated the Changelog and did a minor version bump as asked in #665
I did not find any tests related to sqlState

Test :

var mysql = require('mysql2');
var connection = mysql.createConnection({user: 'xx', password: 'xx', database: 'xx', host: 'xx'});
connection.query("SIGNAL SQLSTATE '02000' SET MESSAGE_TEXT = 'My message', MYSQL_ERRNO = '1999';", function (err, rows) {
  console.log(JSON.stringify(err));
});

Before :

{"errno":1999,"sqlState":"#02000","sqlMessage":"My message"}

After :

{"errno":1999,"sqlState":"02000","sqlMessage":"My message"}

[sidorares]

Thanks!